1. Field of the Invention
This invention relates to electrical conductors and to methods for fabricating electrical conductors which tolerate bending without jeopardizing the physical integrity of the conductive elements therein. More particularly, the present invention relates to the manufacture of superconducting cables capable of being bent to a limited degree to fabricate electrical devices, such as coils for generating electromagnetic fields.
2. Background Art
The recent development of electrically conductive materials which will transfer electricity with previously unknown low amounts of resistance at relatively high operating temperature offers great potential in many electrical areas. Such so-called superconductor materials are capable of passing large currents without generating excessive amounts of heat. Thus, by using superconductor materials, it will be possible to produce intense electromagnetic fields in an efficient and economical manner. To do so requires that coils be designed that incorporate the new superconductor technology.
The material properties of a superconductor present some obstacles. Known high temperature superconductor materials are, generally speaking, brittle ceramic substances which lack the ductility associated with most other metallic conductors and, accordingly, do not have any appreciable tensile strength or flexibility. It happens also that some low temperature superconductors, e.g. niobium tin (Nb.sub.3 Sn), also have these limitations.
The brittleness of known superconductor materials can cause difficulties when the conductors are manipulated. This is so because ceramic materials generally tolerate compressive stresses more readily than tensile stresses. The problem has some unique considerations when the superconductor is deposited on a metallic wire substrate. More particularly, the problem arises from the different stresses which are placed on the superconductor whenever the wire is bent.
In order to avoid excessive tensile stresses when twisting superconductor filaments together to produce filament bundles, extreme care must be taken to not cause sharp bends along the length of each filament. Bending produces compressive stresses on the side of the filament adjacent to the bend which can usually be tolerated without impairing the structural integrity of the superconductor layer. On the other hand, tensile stresses are produced in the ceramic material on the side of the bend opposite from the direction of bending which produce cracks and detract from the conductivity of the superconductor.
While the twisting of superconductor filaments required to produce a superconductor filament bundle can be relatively gradual, the problem of bending in superconductor materials becomes particularly severe when such bundles are to be installed in electrical machinery. There, the severity of bending is frequently dictated by the size and design of the equipment into which the superconductor material is to be incorporated. It is a challenge to handle, install and operate with superconductor materials while preserving their fragile structure.
For example, in the manufacture of electrical coils for producing high-intensity electromagnetic fields, bundles of superconductor filaments must be wound repeatedly in circular courses about rotors, about stators and within magnetic chambers. Generally, the repeated bending of wires in such coils is in the same lateral direction throughout much of the length of the wire or filament bundle. Thus, superconductor filaments on one side of the filament bundle tend to be subjected to potentially damaging amounts of tensile stress. To a degree, the twisting of the superconductor filaments within each fiber bundle helps to prevent this by rotating the position of each superconductor filament within the body of the filament bundle. Nevertheless, it has been found that prior methods for aggregating such filament bundles and packaging them into wires and cables usable in electrical machinery has proved difficult.